1. Field of the Invention
The invention relates to a position detecting means comprising a sensor arrangement for producing at least one sensor signal in a manner dependent on a body moving past the means.
2. Description of Related Art
Conventional position detecting means, see for example German patent publication 19,504,229 A1 and more particularly those designed for detecting a piston in a cylinder, serve to produce a position signal at one or more given piston positions by means of which the display means and/or the control routines are able to be operated and, respectively, implemented. For this purpose the piston is provided with a permanent magnet and as a sensor a magnetic field sensor, more particularly a hall effect sensor, is employed. On moving past the permanent magnet a sine signal is produced in the hall effect, which in a comparator is compared with a predetermined, set reference value. If this reference value is reached or, respectively, exceeded, a position signal is generated. The sensor arrangement is normally arranged to be slid in the longitudinal direction and may be set at any desired positions. Since the position signal must be detected in an extremely accurate manner, it is necessary to provide for complex and accurate adjustment of the sensor arrangement.
One object of the present invention is to produce an exact position signal without complex mechanical setting operations.
This object is attained in accordance with the invention by a position detecting means.
The advantage of the position detecting means of the invention is more particularly that the sensor arrangement only needs to be approximately fitted adjacent to the position to be detected, whereas exact setting may be performed purely electrically by changing the reference value. This is even possible by remote control. This adjustment may be implemented in a simple fashion by moving the movable body, for example a piston, into the desired position in which a position signal is to be produced, the reference value then being changed until the position signal is produced.
A preferred embodiment is such that the body produces a magnetic field or possesses a magnet for producing such a magnetic field and that the sensor arrangement comprises at least one magnetic field sensor, more particularly in the form of a hall effect sensor. In principle the invention is however applicable to other principles of measurement, for example to optical, to infrared or to electromagnetic measurement methods.
In accordance with a preferred development the sensor arrangement comprises two sensors, of which preferably the one produces a sine signal and the other produces a cosine signal proportional to the relative position of the body and the sensor arrangement, a signal processing circuit being provided for conversion of such signals into a signal substantially linearly dependent on the relative position. Such a sine signal and a cosine signal may for example be produced by two magnetic field sensors, which are arranged at an angle of 45 degrees to one another. By combination of the sine signal and the cosine signal it is possible to produce a substantially linearly rising or, respectively, decaying signal, it not being the absolute amplitude but the phase relationship which is relevant for conversion. Accordingly there is an independence from variations in the strength of the magnetic field owing to inaccuracy of magnetic components and lack of homogeneity of the magnet and furthermore temperature dependency of the magnet.
As magnetic field sensors two hall effect sensor bridge circuits are suitable, which are commercially available as self-contained complete components.
The comparator arrangement preferably possesses a window comparator or schmitt-trigger. Therefore it is possible for the start and end of a signal of the position signal to be adjusted. As control means for changing the reference value it is more particularly suitable to employ an electrically adjustable potentiometer or a signal storage means for the reference value, more particularly in the form of a digital counter or a digital memory. The same may either be arranged adjacent to the sensor arrangement and constitute a structural unit with it or however they may be arranged separate from it in space, the adjustment of the control means and/or the setting of the reference value then taking place by remote control by way of an electrical line, more particularly a bus, or in some wireless manner. For the adjustment of the control means or, respectively, for setting the reference value it is possible to provide at least one manual operating element.
The evaluating circuit comprising at least the comparator arrangement may constitute a component group together with the sensor arrangement or it is possible for such evaluating circuit to be separate in space and more particularly be integrated in a program controlled control unit. If an analog comparator arrangement is provided, it is then possible for the digital signal from such a program controlled control unit to be converted by means of a digital/analog converter into an analog signal.